She was the daughter of the Prussian general Hans von Kretschmann. Her grandmother, Baroness Jenny von Gustedt, had been an illegitimate daughter of Jérôme Bonaparte, Napoleon's brother who was King of Westphalia.

Lily Braun was briefly married to the professor of philosophy Georg von Gizycki, who was associated with the Social Democratic Party of Germany (SPD) without however being a member of the party. Together with him she was involved in the ethical movement, which sought to establish a system of morality in place of the traditional religions. After his death, she married in 1896 Heinrich Braun (1854–1927), who was a Social Democratic politician and a publicist.

Lily Braun joined the SPD at an early age and became one of the leaders of the German feminist movement. She belonged to the revisionist opposition within the SPD. Revisionists didn't believe in the theories of historical materialism and believed in the gradual adaptation of society, rather than a socialist revolution.

Lily was heavily influenced by Friedrich Nietzsche and wanted the SPD to focus on the development of personality and individuality instead of levelling everybody. Women should have their own personality and should not have to be only regarded as (future) mothers and wives. She wanted economic freedom for women and the abolition of legal marriage.

Mutterschaft : ein Sammelwerk für die Probleme des Weibes als Mutter (Motherhood: A collection of works on the problems of women as mothers)

Die Liebesbriefe der Marquise (The Marchioness's Loveletters)

Die Frauen und der Krieg (Women and the War)

Im Schatten der Titanen : Erinnerungen an Baronin Jenny von Sustedt (In the Shadow of the Titans: Recollections of Baroness Jenny von Sustedt)(1908) - a biography of Braun's grandmother; the "Titans" of the title were Napoleon Bonaparte, who was von Sustedt's uncle, and Goethe, with whom she came in contact in her Weimar childhood.